Solar cells are photovoltaic components for direct generation of electrical current from sunlight. Due to the growing demand for clean sources of energy, the manufacture of solar cells has expanded dramatically in recent years and continues to expand.
Various types of solar cells exist. Solar cells include absorber layers that absorb photons from sunlight and convert the sunlight to electrical energy. One such absorber layer is a CIGS (Cu(In,Ga)Se2) absorber layer, but other absorber layers are used in other solar cells. The absorber layer and other material layers used in solar cells are formed using various deposition processes that deposit layers of material over a substrate. A molybdenum, Mo, layer is commonly used as the back electrode layer formed either directly on or over the substrate. A molybdenum selenide, MoSe2 layer is often formed over the Mo back electrode and used to establish ohmic contact between the Mo back electrode and the other layers of the solar cell such as the absorber layer. It is important but difficult to control the thickness of the molybdenum selenide layer formed over the molybdenum back contact layer. More particularly, it is difficult to control the thickness of the MoSe2 layer formed using the selenization processes used to introduce selenium to the Mo layer to convert the Mo film to an Mo—Se material. It is difficult to control the degree of selenization and the amount of Mo material converted to MoSe2. When the thickness of the MoSe2 films is too great, higher sheet resistance results and device performance is degraded. Furthermore, thick MoSe2 films also cause adhesion problems between the CIGS or other absorber layer and the underlying Mo back electrode layer.
It would therefore be desirable to form molybdenum selenide layers with accurately controlled thicknesses.